As industrial production methods of organopolysiloxanes, there have so far been known the method of hydrolyzing organohalosilanes and the method of hydrolyzing organoalkoxysilanes.
The former method in which organohalosilanes are used as starting material has an advantage of simple production process, but involves a disadvantage of failing in ensuring the same composition to organopolysiloxanes every time they are produced since the hydrolytic condensation of halosilanes proceeds at such a high speed as to make the control thereof difficult. In particular, the former method has high possibility of causing gelation or generating microgel when trifunctional or tetrafunctional silanes are used in a large amount with the intention of producing organopolysiloxanes to be used as curable resin. Therefore, that method is far from being a good method.
On the other hand, the latter method in which organoalkoxysilanes containing alkoxy groups as hydrolyzable groups are used is almost free from the foregoing disadvantage since the hydrolysis of organoalkoxysilanes proceeds at lower speed than that of halosilanes. Thus, the latter method enabled for the first time organopolysiloxanes suitable for use as curable resins to be produced in an industrial scale from large amounts of trifunctional and tetrafunctional silanes.
In order to meet a wide variety of performances required of organopolysiloxanes to be used as curable resins, it is important for a method of producing them to ensure a wide range of possibility in designing the product performance as well as the prevention of gelation and microgel generation and the high reproducibility of organopolysiloxanes' performance as the products. In this respect, however, any particular improvements have not yet been introduced in the conventional method using as starting material alkoxysilanes whose alkoxy groups are the same.
Basically important factors in the performance design of organopolysiloxanes to be used as curable resins are the kinds and the contents of functional groups introduced therein and the average molecular weight thereof. These factors have decisive influences upon workability at the time of using curable resins (including a curing speed and a drying time) and performances of the cured matter (including hardness, flexibility, mechanical strength, chemical resistance and so on). Accordingly, it is apparent from the viewpoint of product design that favorable methods are those which not only enable organopolysiloxanes to be designed so as to contain functional groups chosen from a wide variety of species in various contents and have their average molecular weight in a wide range, but also can ensure good reproducibility in production of the intended organopolysiloxanes.
If organohalosilanes are used as starting material, however, it is meaningless to control the content of hydrolyzable functional groups. This is because halogen atoms are unsuitable for the functional groups for curing. If alkoxysilanes are used as starting material, on the other hand, it is possible to control the content of remaining functional groups by adjusting the quantity of water added for hydrolysis. However, the controllable range therein is narrow and the organopolysiloxanes obtained are restricted in their average molecular weight.
Controlling the average molecular weight can be accomplished only by the control of the reaction time in the hydrolysis condensation. On the other hand, the reaction condition including the amount of a hydrolytic condensation catalyst used, the siloxane condensation and the reaction temperature enables the adjustment of the reaction rate in the hydrolysis condensation.
The rate of the hydrolytic condensation reaction is however affected by various factors, and so the accuracy of the average molecular weight control by the reaction time becomes low. In addition, the controllable range is limited since the generation of microgel and the gelation must be avoided. Especially in the case of adopting the method of hydrolyzing organohalosilanes high in hydrolytic condensation rate, the condensation product obtained only can have its average molecular weight within a very limited range even if the reaction condition is changed variously.
On the other hand, the production of organopolysiloxanes from difunctional organosilanes does not encounter the above-described problems, namely the difficulties in controlling the average molecular weight, the microgel generation and the gelation. Such being the case, there was developed the hydrolytic condensation method in which organosilanes were hydrolyzed using a reduced amount of water to increase the proportion of functional groups remaining unhydrolyzed and thereby to liken the resulting silanes to difunctional organosilanes. This method is useful for prevention of microgel generation and gelation, but actually obtained products contain low molecular weight compounds in large quantities. Consequently, the products are liquids low in viscosity and broad in molecular weight distribution, and so they are unsuitable for curable resins.
Thus, the conventional production methods described above are unsatisfactory with respect to the control of both functional group content and average molecular weight, which are basically important factors for the product design of organopolysiloxanes to be used as curable resins.
In general a wide variety of performances are required of organopolysiloxanes for curable resin use. Therefore, there has so far been adopted a means of mixing plural kinds of organoalkoxysilanes and subjecting them to simultaneous hydrolysis condensation. However, the means is unsuccessful in making a mixture of organoalkoxysilanes as a starting material be brought in the organopolysiloxane product as the composition of the starting material is kept. In other words, it fails in production of organopolysiloxanes in accordance with the product design.
In case the starting material comprises tetramethoxysilane and methyltrimethoxysilane which are high in hydrolytic condensation rate, for instance, their hydrolysis condensates are concentrated in a high molecular weight component of the product, thereby deteriorating the storage stability and the pot life of the product. If the starting material comprises dimethyldialkoxysilanes which are comparatively low in hydrolytic condensation rate, on the other hand, their hydrolysis condensates constitute an oil component, and so the product obtained comes to have drawbacks such that it cannot ensure sufficient softness in the high molecular weight component and undergoes deterioration in film forming ability.
With the intention of solving the above-described problems, there has been proposed a method of properly selecting the species of alkoxy groups introduced as hydrolyzable groups into an organosilane to adjust the hydrolysis rate. Specifically, the method consists in selecting an organosilane having a low hydrolytic condensation rate, e.g., tetrabutoxysilane or methyltripropoxysilane, as a starting material, but using neither tetramethoxysilane nor methyltrimethoxysilane which has a high hydrolytic condensation rate.
In case the weight of alkoxy groups forms a large proportion of the molecular weight of a starting material used, as in the above-cited alkoxysilanes, the organopolysiloxanes obtained are decreased in silicon atom content. In particular, disadvantages which the low silicon content entails are serious in the case of using tetrafunctional and trifunctional silanes. Further, starting materials containing alkoxy groups other than methoxy group are expensive, and so they raise the cost of production. Accordingly, such a production method is inadequate for the industrial purpose.
In another method of using dimethyldialkoxysilanes which are comparatively low in hydrolytic condensation rate, methoxy groups are generally used as the alkoxy groups and, as silane compounds having higher hydrolytic condensation rates which are to be used together therewith, it is necessary to choose them from expensive silanes having, e.g., acetoxy, oxime, amino or isopropenoxy groups. Therefore, this method also is unsuitable for an industrial production method.
Thus, there have not yet been found any industrially useful methods capable of solving the aforementioned problems of the simultaneous hydrolysis condensation reaction.
In general, organopolysiloxanes used in a curable resin composition contain hydrolyzable groups, such as alkoxy groups, or silanol groups as functional groups for curing. Since the organopolysiloxanes containing hydrolyzable groups as functional groups are not cross-linked in the absence of water, not only they have high storage stability, but also they require no particular treatment, e.g., heating at a high temperature, for curing because they are cross-linked by absorbing moisture upon exposure to the air. In this respect, such organopolysiloxanes are superior to those containing silanol groups as functional groups. Therefore, they are used as room temperature curable resins for various purposes.
In the organopolysiloxanes used as room temperature curable resins, as described above, the content of functional groups and the average molecular weight are important factors determining the product performance. In addition to these factor, what kinds of functional groups are introduced therein is also important.
More specifically, different kinds of functional groups differ in hydrolytic condensation rate, and so the curability of organopolysiloxanes depends largely upon the kinds of functional groups they contain.
When organopolysiloxanes have too high curability, not only a complicated control is required for preventing them from deteriorating upon storage, but also a serious workability problem is caused, e.g., due to an increase in viscosity or gelation during mixing with other ingredients and painting or coating operation.
When they have too low curability, on the other hand, the coatings thereof are inferior in drying speed and requires a long time for curing. Therefore, they are at a disadvantage from an industrial point of view.
As another reason why the kinds of functional groups are important, there can be adduced a fact that functional groups remain in the cured matter and the properties of the cured matter are influenced by the kinds of the functional groups remaining therein.
For instance, if alkyl moiety-containing functional groups remain in cured matter, water repellency and staining resistance are imparted to the cured matter. If nitrogen-containing functional groups remain in cured matter, they can sometime enhance adhesion or adhering properties of the cured matter to a substrate or confer antistatic properties on the cured matter.
If functional groups remain in cured matter in excess of necessity, however, they have bad influences on water resistance and weather resistance of the cured matter.
In producing organopolysiloxanes used as room temperature curable resins, therefore, it is desirable to adopt a production method which allows a free choice also in the kind of functional groups. The production method which is at an advantage in this point comprises two steps such that organopolysiloxanes containing silanol groups are firstly produced and then, in the presence or absence of a catalyst, the silanol groups are reacted with one or more compounds chosen from a group consisting of alcohols, carboxylic acids, oximes and organosilanes containing at least two hydrolyzable groups such as alkoxy groups, carboxyl groups, oxime groups, etc. so that the desired functional groups may be introduced in the organopolysiloxanes, thereby achieving the introduction of hydrolyzable groups.
However, the method described above has high possibility of causing gelation, microgel generation or increase in viscosity during reaction, because the condensation of silanol groups also proceeds therein.
In order to remove such drawbacks from the foregoing method, it is possible to add excessive amounts of alcohols, carboxylic acids, oximes and organosilanes containing at least two hydrolyzable groups, but an additional step for removing the organosilanes added in excess becomes necessary. Therefore, such a modification is at a disadvantage in production cost.
If the organosilanes added in excess cannot be removed, on the other hand, they exert unfavorable influences upon the properties of cured matter.
In case the gelation, microgel generation and the increase in viscosity are prevented by adopting a mild reaction condition, unreacted silanol groups remain to deteriorate the storage stability.
As still another method for producing organopolysiloxanes used as room temperature curable resin, there is known a method of using organoalkoxysilanes as a starting material and performing hydrolytic condensation under a condition that a sufficient proportion of the alkoxy groups may remain as functional group. Such a method, however, can allow a limited choice in the species of alkoxy group.
Further, the above-described two methods for production of organopolysiloxanes used as room temperature curable resin have also a drawback to the important factor pointed out hereinbefore, that is, the control of the content of functional groups and the average molecular weight. Thus, it can be said that no methods effective for solving the aforementioned problems have yet been proposed.